What is the CIA document mentioned in Dan Brown's The Lost Symbol?

Question

I've searched for the phrases "It's buried out there somewhere", "cia 1991 portal document", etc. and all the results point to the Kryptos sculpture whose cleartext contains the buried phrase, but it doesn't have any mention of a portal.

Answer 1

It may indeed relate to Kryptos, specifically its solution. According to this page:

After completion of the sculpture Sanborn was required to give the answer in a sealed envelope to the then CIA director William Webster. In 1999 Webster confirmed this fact in an interview to the New York Times, saying that the solution is “philosiphical and obscure”.

Sanborn however claims that the envelope he gave Webster does not contain the complete solution. This fact has also been cleverly highlighted by Dan Brown in his novel ‘The Symbol’.

Dan Brown is infamous for mixing fact with fiction, and claiming that it's all fact, as per the TV Tropes entry formerly named "Dan Browned".